Gas mixtures containing hydrogen sulfide are called "sour" by those skilled in this art. There are many sour natural gas wells whose production is not large enough to justify the cost of a conventional amine system using monoethanolamine in solution with water.
The removal of relatively small amounts of hydrogen sulfide from natural gas without the simultaneous removal of carbon dioxide can be accomplished with the iron sponge process. The sour gas is contacted with hydrated ferric oxide to form ferric sulfide. If regenerated by exposure to air, the ferric sulfide is oxidized to sulfur and ferric oxide which can then be reused. This reaction-regeneration process cannot be repeated indefinitely because the surface of the ferric oxide becomes coated with sulfur.
Shortcomings of the iron sponge process include: (1) incomplete conversion of ferric oxide to ferric sulfide, (2) the pyrophoric nature of ferric sulfide, (3) the labor required to prepare the iron sponge, and (4) the time used to remove the spent iron sponge, particularly when regenerated, and recharge the reaction tower.
It is well known that zinc sulfide is very insoluble. Aqueous solutions of zinc salts combine with hydrogen sulfide to form zinc sulfide. However, as the hydrogen sulfide is absorbed, the pH of the solution decreases, and the point is soon reached where the effectiveness of the solution to absorb hydrogen sulfide is seriously impaired and the danger of corrosion of the metal container increases.
To date, provision, as disclosed in the prior art (eg. U.S. Pat. Nos. 2,378,689 and 2,641,527) to prevent this deleterious decrease in the pH or to replinish the metallic salt as it is consumed, is to add a basic zinc compound. However, these basic zinc compounds are insoluble, have a high specific gravity, tend to agglomerate and settle, and often become coated with sulfide. These phenomena preclude the stoichiometric usage of the basic zinc compound.